1. Introduction
In database management, foreign keys are an important constraint used to ensure the consistency and integrity of data. However, in some cases we may need to delete or modify foreign keys. This article will introduce how to use the petl library in Python to delete the foreign keys of all tables in PostgreSQL, migrate the data, and re-establish the foreign keys.
2. Install petl and psycopg2
First, we need to install the petl and psycopg2 libraries. Enter the following command on the command line to install:
pip install petl psycopg2
3. Connect to the source database
Before starting the data migration, we need to connect to the source database. We can use the psycopg2 library to achieve this. Here's a simple example:
import psycopg2
from petl import load, dump
# 连接到源数据库
conn = psycopg2.connect(database="source_db", user="user", password="password", host="host", port="port")
cur = conn.cursor()
In this example, we first imported the psycopg2 library and petl library. We then used psycopg2's connect function to connect to the database named 'source_db' and specified the username, password, host and port. Finally, we create a cursor object for executing SQL queries.
4. Get the names and foreign key information of all tables
Before we start reading data, we need to get the names and foreign key information of all tables in the source database. We can achieve this using the following SQL query:
SELECT table_name, column_name, referenced_table_name, referenced_column_name
FROM information_schema.key_column_usage
WHERE constraint_name NOT LIKE '%FOREIGN KEY%';
We can use psycopg2’s execute function to execute this query and save the results to a variable:
cur.execute("SELECT table_name, column_name, referenced_table_name, referenced_column_name FROM information_schema.key_column_usage WHERE constraint_name NOT LIKE '%FOREIGN KEY%';")
for row in cur.fetchall():
print(row)
In this example, we first execute a SQL query that selects all constraint names, table names, column names, and referenced table names and column names that do not contain "FOREIGN KEY" from the information_schema.key_column_usage table. We then iterate through the query results and print each row.
5. Delete foreign keys and migrate data
Once we have the names and foreign key information for all tables, we can start removing the foreign keys and migrating the data. We can achieve this using petl's fromcsv function. Here's a simple example:
for table in tables:
# 从源数据库读取表的数据和外键信息
data = load(f"SELECT *, '{
table}'::regclass::text AS table_name FROM {
table}.\"%\";")
foreign_keys = load(f"SELECT * FROM {
table}_key;")
# 删除外键并迁移数据到新表
for fk in foreign_keys:
if fk[3] != 'PRIMARY': # 如果不是主键外键，才删除外键并迁移数据到新表
cur.execute(f"ALTER TABLE {
table} DROP CONSTRAINT {
fk[0]};")
data = data.join(data.loc[:, fk[1]:], on=fk[0], how='left') # 删除外键后，使用left join将数据迁移到新表
data = data.rename({
fk[1]: f"{
table}_new_{
table_name}"}) # 重命名列名以区分旧表和新表的数据
dump(data, f"{
table}_new_{
table_name}.csv", sep=',', header=True) # 将数据导出到CSV文件以备后续导入到新表中使用（可选）
In this example, we first iterate through the names of all tables. Then, for each table, we use the load function to read the table's data and foreign key information from the source database. Next, we iterate through all foreign key information. If the foreign key is not the primary key or foreign key, we use the ALTER TABLE statement to delete the foreign key. Then, we use left join to migrate the data to a new table, and use the rename function to rename the column names to distinguish the data of the old table and the new table. Finally, we export the data to a CSV file for subsequent import into a new table (optional).